Compatibility between cements and superplasticizers in high-performance concrete: Influence of alkali content in cement and of the molecular weight of PNS on the properties of cement pastes and concretes

The aim of this study is to evaluate the effects of alkali content in cement and the molecular weight of PNS on the properties of cement pastes and concretes. Moreover, the dispersing mechanism of the PNS superplasticizer and the role of alkali in the dispersing mechanism are also proposed in this study. Six cements, having a wide range of C₃A and Na₂O equivalent contents, have been carefully selected for this study. Three commercial PNS superplasticizers, having different molecular weights, were selected and were characterized by their molecular weight distribution.; The results obtained from the study of the effects of alkali content show that there is an optimum content of soluble alkali with respect to fluidity and fluidity loss, which was found to be 0.4–0.5% Na₂O soluble equivalent in the cements. It was found that there is not a close relation between the total alkali and the soluble alkali content measured in the cement pastes having a W/C = 0.35. Therefore, it is important to measure the soluble alkali content rather than the total alkali content. It was found that the three incompatible cements containing a low amount of soluble alkalies have a tendency to adsorb a high amount of PNS superplasticizer, while the three compatible cements containing a higher amount of soluble alkalies do not, leaving a higher amount of PNS superplasticizer in solution.; It was found that the influence of the molecular weight of PNS superplasticizer on the rheological behavior of cement pastes depends greatly on the alkali content in the cement; relatively high-molecular-weight PNS is more effective in fluidizing pastes made with high-alkali cements than low-molecular-weight PNS, however, PNS molecular weight has little effect in fluidizing pastes made with low-alkali cements. However, in the study on concretes, the high-molecular weight PNS is always better in improving the fluidity of concretes than low-molecular weight PNS. The fluidity of cement pastes containing a high molecular weight PNS is more affected by the addition of Na₂SO₄ because the amount of large molecules remaining in solution is determined by the alkali content in the cement. However, low-molecular-weight PNS does not affect greatly the fluidity of cement paste when some Na₂SO₄ is added. (Abstract shortened by UMI.)...